The present invention relates generally to fiber-optic communication systems. More specifically, the present invention relates to applying error-correction codes to improve spectral efficiency in wavelength-division multiplexing (WDM) fiber optic communication systems.
Error correction codes have been applied with varying degrees of success to various types of optical systems, including WDM systems, in attempts to improve system performance. First, for example, error codes have been applied to provide a performance margin to compensate for minute device and subsystem flaws. See, e.g., Optical Fiber Telecommunications IIIA, xc2xa73.6.1, Ed. Kaminow and Koch, Academic Press, 1997. As a result, less expensive and more mass-producible optoelectronics can be used, and the overall system lifetime can be extended as devices slowly degrade.
Second, error-correction codes also have been applied to reduce the required optical signal power. See, e.g., Optical Fiber Telecommunications IIIA, xc2xa73.6.2. Because error correction codes allow an optical system to operate at lower optical signal powers, the number of optical amplifiers necessary in a long distance fiber-optic link can be reduced. This is particularly important in, for example, transoceanic fiber-optic links traversing long distances through difficult-to-access environments that are hostile to system equipment that rely on electronic components. Such transoceanic fiber-optic links often use optical amplification to increase the optical-signal power rather than electronic regeneration which is less reliable and more expensive to maintain in such inaccessible and hostile environments. By applying error-correction codes to reduce the number of required optical amplifiers for a given fiber-optic link, the overall cost of installing that link is reduced and the overall reliability of that link is improved.
Third, error correction codes also have been applied to shift the operation point of WDM channels, thereby allowing more cross talk and nonlinearities. See, e.g., Optical Fiber Telecommunications IIIA, xc2xa73.6.3; Livas, J. C., et al., xe2x80x9cForward error correction in a 1 Gbit/s/channel wavelength-division multiplexed system,xe2x80x9d IEEE/LEOS Summer Topical Meeting on Optical Networks and their Enabling Technologies, July 1994. As FIG. 1 illustrates, the bit-error rate is an exponential function of the channel density. In other words, as the channel spacing for a WDM system is decreased, the bit-error rate of the receiver system increases relatively slightly until some point (i.e., the xe2x80x9coperation pointxe2x80x9d labeled in FIG. 1 as 10a) at which the bit-error rate dramatically increases. After the operation point for a system has been determined, error correction codes have been applied to shift this operation point (labeled in FIG. 1 as 10b) so that WDM channels can be spaced closer together without dramatically increasing the bit-error rate. This application of error-correction codes to a WDM system has yielded an increase in channel packing density of a factor 2.5 for an error correction code with a coding gain of 3.7 dB.
These known applications have applied error-correction codes to WDM system in only a limited sense with limited results. Known applications of error correction codes have not provided the fullest performance benefits possible.
Embodiments of the present invention address the shortcomings of the known applications of error-correction codes. One embodiment of the present invention is a method for designing an optical system. The optical system launches optical signals modulated with data into a fiber link having a property of inducing nonlinear distortion of the optical signals as a function of signal power of the optical signals and distance traversed in the fiber link. A Q-factor curve for the fiber link is determined as a function of the signal power. A signal power is preselected based on the Q-factor curve. The preselected signal power is associated with a set of channels. A coding gain is preselected based on the preselected signal power and a desired channel spacing for the set of channels.
Another embodiment is a method for launching optical signals modified with data into a fiber link. The data is error-correction coded with a coding gain to produce encoded data signals. The encoded data signals is multiplexed to produce channels each of which carries optical signals based on the encoded data signals. A preselected coding gain is based on a preselected signal power associated with the channels and a desired channel spacing for the channels